In recent years, radiation imaging apparatuses using a flat panel detector (FPD) formed of semiconductor materials have been put into practical use as imaging apparatuses to be used for X-ray based medical imaging diagnosis or non-destructive testing. Hereinafter, flat panel detectors will be generally referred to as “detectors”. In, for example, medical imaging diagnosis, such radiation imaging apparatuses have been used as digital imaging apparatuses for still image radiography such as general imaging or moving image radiography such as fluoroscopic imaging. Known detectors include an indirect conversion detector using a conversion element including, in combination, a photoelectric conversion element made of amorphous silicon and a wavelength conversion member that converts radiation into light of a wavelength band detectable by the photoelectric conversion element. Other known detectors include a direct conversion detector using a conversion element that is made of a material such as amorphous selenium and that converts radiation directly into charge.
In such imaging apparatuses, for a conversion element having a semiconductor layer made of amorphous semiconductor, a dangling bond or defect in the semiconductor layer acts as a trap level. When imaging is performed a plurality of times, charge generated by radiation or light applied in the preceding imaging operation may be trapped in the trap level. In this case, a so-called afterimage (lag), which is affected by the charge trapped in an image obtained in the subsequent imaging operation, may occur in the image obtained in the subsequent imaging operation. The afterimage may be prevented by extending the time between the preceding imaging operation and the subsequent imaging operation (hereinafter referred to as the imaging operation interval); however, if the imaging operation interval is increased, usability is impaired. For this reason, an imaging apparatus is demanded which prevents an afterimage from occurring in the subsequent imaging while reducing the imaging operation interval.
Patent Literature (PTL) documents 1 and 2 disclose a reset operation of supplying a voltage different from that in the imaging operation to a conversion element such as a photodiode or an MIS photoelectric conversion element during a plurality of imaging operations in order to prevent an afterimage. Specifically, in PTL 1, a reverse voltage larger or smaller than a reverse voltage to a photodiode during an imaging operation, or a forward voltage during a reset operation is supplied to the photodiode during a reset operation. In PTL 2, a voltage different from that during an imaging operation is supplied to an MIS photoelectric conversion element so that both electrodes of the MIS photoelectric conversion element are grounded during a sleep (reset) operation. It is disclosed in PTLs 1 and 2 that charge which may cause an afterimage is removed from the conversion element by using the reset operation.